1. Field of the Invention
The present invention relates to electroconductive magnetic carrier particles, a developer comprising the carrier particles and an electrophotographic image formation method using such a developer for use with a printer, facsimile apparatus and digital copying apparatus.
2. Discussion of Background
According to the electrophotographic image formation method based on the Carlson process, which is now widely employed, image formation is basically carried out in such a manner that the surface of a photoconductor is uniformly charged to a predetermined polarity and the photoconductor thus charged is selectively exposed to the original light images to form latent electrostatic images on the photoconductor. Then, the latent electrostatic images are developed with a developer, so that visible toner images can be obtained on the photoconductor. The visible toner images are then transferred to a sheet of an image-receiving medium and fixed thereon.
On the other hand, many proposals on the image formation method not using the Carlson process, but using the rear side exposure system have been reported, for example, in The Journal of the Institute of Image Electronics Engineers of Japan vol. 16, (5), 306 (1987); and Japanese Laid-Open Patent Applications 61-149968, 63-10071 and 63-214781, by which rear side exposure system the image formation apparatus can be made compact and the image formation process can be made simple.
In the rear side exposure system, the surface of the photoconductor is provided with a developer to form a developer resident portion, through which the photoconductor is subjected to a cleaning operation, and the photoconductor is uniformly charged. The light images are applied to the photoconductor from the rear side thereof and the latent images formed on the surface of the photoconductor are simultaneously developed into toner images with the developer.
However, there are too many difficult problems in the rear side exposure system to put it into practice. More specifically, the requirements for each function in the rear side exposure system are made extremely severe because it is necessary to inject the electric charge in a sufficient amount for the development into the photoconductor through the developer accumulated in the developer resident portion and to form sharp and stable toner images on the photoconductor by development at a relatively small developer resident portion.
In addition, it is necessary to impart the electroconductivity to the developer since the electric charge is injected into the photoconductor through the developer. Therefore, when a developer to be employed is a one-component type developer, an electroconductive magnetic toner is essentially required. The toner image thus formed on the photoconductor cannot be transferred to a sheet of plain paper by the electrostatic image transfer method such as corona transfer or bias roller transfer. As a result, only a sheet of paper with high resistivity can be used in this system.
The method of forming a multi-color recording image on a sheet of plain paper by the rear side exposure system is disclosed in Japanese Patent Publication 60-59592. In this method, however, since a photoconductor is prepared by overlaying an insulating layer on a photoconductive layer, the photoconductor cannot stand the repetition of formation of multi-color images thereon. To solve this problem, it is proposed that the residual latent image formed on the photoconductor be erased by application thereto of a transfer electrical field. This proposal is still insufficient in practice for obtaining clear images over an extended period of time.
As in the Journal of the Institute of Electrophotography Engineers of Japan vol. 27, No. 3, p.442 (1988) and Japanese Laid-Open Patent Application 61-46961, the image formation can be achieved by the rear side exposure and the simultaneous development system, with the application of a charging bias and a development bias to a photoconductor, having counter polarities, using a two-component type developer comprising iron carrier particles with a resistivity of 10.sup.4 to 10.sup.8 .OMEGA..cm and magnetic toner particles with insulating properties.
However, when the above-mentioned image formation method is applied to the practically-used copying apparatus, it is difficult to control the image formation system for obtaining a clear image over an extended period of time, and in addition, the structure of the apparatus necessarily becomes complicated.
Metallic particles such as copper particles are conventionally known as the electroconductive carrier in addition to the above-mentioned iron particles. However, when the particle size of such metallic particles is less than a definite value, the metallic particles conspicuously tend to cause aggregation, and at the same time, the surface of the metallic particles is oxidized, so that the resistivity thereof is increased. Therefore, the metallic carrier particles which constantly show low resistivity cannot be obtained, and it is difficult to produce metallic carrier particles with a particle diameter of as small as about 20 .mu.m. Furthermore, since the density of the metallic carrier particles is too large, the toner concentration in a developer cannot be increased, thereby lowering the image density.
In addition to the iron particles with a density of 7.8 g/cm.sup.3, magnetite particles with a density of 5.2 g/cm.sup.3 and ferrite particles with a density of 5.6 g/cm.sup.3 are conventionally known as the magnetic carrier particles. Any of the above-mentioned conventional magnetic carrier has extremely large density, so that a sufficient toner concentration cannot be obtained.
There is proposed in National Technical Report, Vol. 28, No. 4, Aug., 1982, an image formation system employing a resin carrier in which magnetic particles are dispersed in a binder resin. In this system, the resin carrier is electrically charged to a polarity of opposite to that of a toner to be employed. To produce images with an image density of 1.0 or more, it is required to set the surface potential of the employed photoconductive drum at 500 V or more. However, when the image formation is carried out with the surface potential of the photoconductive drum being increased to a high level as previously stated, the carrier is also attracted and transferred to the photoconductive drum together with the toner (hereinafter referred to as "carrier attraction phenomenon"). It is necessary to increase the particle size of the carrier to prevent the occurrence of the above-mentioned carrier attraction phenomenon. However, there are many problems in extremely increasing the particle size of the carrier from the viewpoints of guarantee of image quality, injection of electric charge into the photoconductive drum and transporting characteristics of the carrier.
Furthermore, the image formation methods by use of a magnetic carrier prepared by dispersing a magnetic material in a binder resin have been proposed. For example, a developer comprising the above-mentioned magnetic carrier and an electrically insulating non-magnetic toner is employed in Japanese Laid-Open Patent Applications 53-33152 and 55-41450; and a developer comprising the above-mentioned magnetic carrier and an electrically insulating magnetic toner is employed in Japanese Laid-Open Patent Applications 53-33152, 53-33633 and 53-35546. Furthermore, it has also been reported that a charge controlling agent such as nigrosine dye, resin particles, and titanium dioxide particles are fixed on the surface of a magnetic resin carrier particle prepared by dispersing a magnetic material in a binder resin. In these disclosures, however, the carrier component in a developer has insulating properties and the development is carried out by the conventional Carlson process.
In addition, in the case where the electroconductive magnetic carrier is used for development, it is necessary to impart magnetism to the toner to some extent to prevent the scattering of the toner in the image formation apparatus. However, it is difficult to produce a color image with sufficient transparency using the magnetic toner provided with the magnetic material. This involves many problems in coping with the formation of clear color images.